This invention relates to improvements in semiconductor devices. The invention is particularly applicable to gallium arsenide/aluminum gallium arsenide heterostructure devices that are useful as light emitters, and can also be used as light detectors or field effect devices.
Various types of semiconductor light emitters are described in the prior art. In a gallium arsenide homojunction light emitter, electrons are injected across a pn junction, combine with holes, and give up excess energy by emitting light at a wavelength characteristic of the material. In a so-called double heterojunction light emitter, fabricated, for example, using a gallium arsenide/aluminum gallium arsenide material system, a pair of relatively wide bandgap layers (aluminum gallium arsenide) of opposite conductivity type are sandwiched around an active region (gallium arsenide). The interfaces between the active region and the wide bandgap layers form a pair of heterojunctions. These heterojunctions effectively provide carrier confinement and optical confinement. The devices are generally used as light emitting diodes or lasers, and may be energized using an electrical current or optical pumping.
There are a number of practical constraints which affect operation and performance of semiconductor light emitting devices. For example, relatively high current densities may be necessary to achieve a desired level of light emission or laser action. Temperature is a significant consideration and, while it is desirable to have devices that work at room temperature, lower temperature operation is often required if continuous operation is desired. The wavelength of the radiation produced is also significant, and is not generally a matter of flexible choice. The wavelength of radiation generated by conventional double heterojunction devices is a function of the bandgap of the active region. Within limits, the wavelength of the radiation produced can be changed by altering the composition of the active region.
Much of the current interest in semiconductor light emitters (especially lasers) is concerned with sources at wavelengths longer than the visible, e.g., Al.sub.x Ga.sub.1-x As-GaAs or InP-In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z double heterostructures. However, certain proposed applications, for example in photocopying or video-disk recording, provide ample reason for interest in visible-spectrum heterostructure lasers. The development of lasers and other light emitters in the visible portion of the spectrum has been limited, however, because of various difficulties in working with high-gap III-V alloys. In the case of high-gap In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z, the substrates employed (commercial GaAsP LED substrates) are not lattice-matched throughout and are of relatively poor quality. For the case of high-gap Al.sub.x Ga.sub.1-x As there is, in the direct-gap alloy range (x&lt;x.sub.c .about.0.45), a limit to the heterobarrier height or energy-gap discontinuity between the active region and the confining layers.
Visible-spectrum semiconductor lasers have been constructed which employ a GaAs quantum well in an Al.sub.x Ga.sub.1-x As-GaAs device. These quantum-well heterostructures can tend to be relatively inefficient laser sources, however, because of their broad spontaneous spectra and thus wasted recombination. If an attempt is made to effect an improvement in Al.sub.x Ga.sub.1-x As-GaAs quantum well heterojunction devices by shifting the n=1 states (electrons, heavy holes, light holes) to higher energies by reducing the GaAs quantum well size to less than 50 angstrom thickness, multiple wells need be employed to yield a sufficiently large total active region and to collect the injected carriers but, unfortunately, lower-energy recombination and a broad recombination radiation spectrum are still observed.
It is an object of the present invention to improve upon the deficiencies observed in gallium arsenide/aluminum gallium heterostructures and particularly in gallium arsenide/aluminum gallium arsenide quantum well heterostructure devices.